Emulsion pressure-sensitive adhesive polymers exhibiting excellent room- and low-temperature performance

ABSTRACT

There is provided inherently tacky, emulsion pressure-sensitive adhesive polymers comprising about 35 to about 65 percent by weight alkyl acrylates, about 15 to about 35 percent vinyl esters, about 20 to about 35 percent by weight diesters of a dicarboxylic acid, and up to about 5 percent by weight of an unsaturated carboxylic acid. There is preferably included in the monomers a reactive surfactant and a chelating monomer, with or without a chain transfer agent. The preferred polymers have a glass transition temperature less than about -30° C. and a gel content of about 50 to 70 percent by weight.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 07/567,141 filed Aug. 14,1990, now U.S. Pat. No. 5,164,444, which is a continuation-in-part ofapplication Ser. No. 07/393,970, filed Aug. 14, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to polymers which are inherently tackyand, as formed, are functional as pressure-sensitive adhesives. Moreparticularly, the adhesives of the instant invention have excellentroom- and low-temperature performance and provide an ecologically safereplacement for solvent adhesives and many acrylic- and rubber-basedemulsion adhesives. The adhesives of the instant invention are preparedby emulsion polymerization.

Adhesives are provided as solvent polymers, bulk polymers and emulsionpolymers. Some exist as pressure-sensitive adhesives, while othersrequire tackification to achieve this end.

In respect of the instant invention, U.S. Pat. No. 3,275,589 toAlexander, et al. pertains to an adhesive for adhering polyolefin andsimilar hydrophobic polymers to themselves and to other materials. Thepolymers disclosed are tacky, but not pressure-sensitive adhesives andare obtained by the polymerization of 100 parts by weight of a monomermixture comprising 30 to 40 parts by weight vinyl acetate and,correspondingly, 60 to 70 parts by weight of (a) a monoalkyl ester of analpha beta ethylenically unsaturated monocarboxylic acid such as 2-ethylhexyl acrylate, or (b) a dialkyl ester of an alpha beta ethylenicallyunsaturated dicarboxylic acid such as 2-ethyl hexyl maleate, or (c) amixture of the two. When three monomers are employed, the threeillustrated include vinyl acetate, di-2-ethyl hexyl maleate and 2-ethylhexyl acrylate.

U.S. Pat. Nos. 4,507,429, 4,694,056, and 4,725,639 to Lenney, andassigned to Air Products, Inc., pertain to a pressure-sensitive adhesivecomposition, and the products and method of making them, comprising apolymer of an acrylic ester and/or vinyl ester, an olefinicallyunsaturated carboxylic comonomer, and a polyolefinically unsaturatedcopolymerizable monomer employed to enhance the adhesive strength of thepolymer. The polymer is formed by an aqueous emulsion polymerization inthe presence of a stabilizer system containing hydroxy propylmethylcellulose and an ethoxylated acetylenic glycol. Product glasstransition temperature may be as high as -15° C., limiting utility atreduced temperature.

U.S. Pat. Nos. 4,753,846, 4,826,938 and 4,829,139, all to Mudge andassigned to National Starch & Chemical Corporation, pertain to anethylene containing pressure-sensitive adhesive produced byhigh-pressure emulsion polymerization in which the adhesive comprises apolymer of 30 to 70 percent by weight of a vinyl ester of an alkyl acidsuch as vinyl acetate, 10 to 30 percent by weight of ethylene, 20 to 40percent by weight of di-2-ethyl hexyl maleate, and 1 to 10 percent byweight of a monocarboxylic acid such as acrylic acid. Again, glasstransition temperature may be as high as -25° C., also limitinglow-temperature utility.

U.S. Pat. No. 4,322,576 assigned to Wacker Chemie GmbH also pertains toethylene containing vinyl acrylate polymers also produced and formed byhigh-pressure emulsion polymerization. Dialkyl esters of dicarboxylicacids are not employed as monomers.

We have sought to develop pressure-sensitive adhesives of controlledproperties which serve to supplant a multitude of solution and emulsionpolymers by having broad utility and excellent adhesive properties atambient and low temperatures. This is the subject of the instantinvention.

SUMMARY OF THE INVENTION

According to the present invention, there is provided inherently tackyacrylic emulsion adhesive polymers having excellent adhesion to a widevariety of surfaces ranging from polar, relatively high energy surfacessuch as stainless steel to nonpolar, relatively low energy surfaces suchas polyethylene and to difficult-to-bond surfaces such as corrugatedboard. Moreover, cohesion and adhesion at low temperatures is excellentand, when part of a laminate stock such as pressure-sensitive adhesivelabel stock, provides excellent high-speed converting characteristicssuch as die cutting, matrix stripping and fan folding. Superiorproperties are obtained even at lower-than-normal coat weights. In sum,the adhesive polymers are broad-based and serve to replace manysolvent-based adhesives on an ecologically safe basis as well as manyemulsion-based adhesives, thus serving a variety of markets.

The inherently tacky, emulsion pressure-sensitive adhesive polymers ofthe instant invention comprise, on a polymerized basis and based on thetotal weight of the polymer, at least one alkyl acrylate containing fromabout 4 to about 8 carbon atoms in the alkyl group, preferably 2-ethylhexyl acrylate, said alkyl acrylate present in a total amount of fromabout 35 to about 60 percent by weight; at least one vinyl estercontaining from 2 to about 16 carbon atoms in the alkyl chain of theacid, preferably vinyl acetate, said vinyl ester present in a totalamount of from about 15 to about 35 percent by weight; at least onediester of a dicarboxylic acid wherein each alkyl group of the diesterindependently contains from about 6 to about 12 carbon atoms, withdi-2-ethyl hexyl maleate or di-2-ethyl hexyl fumarate being preferred,said diesters being present in a total amount of from about 20 to about40 percent by weight; up to about 5 percent by weight, preferably about1 to 3 percent, of an unsaturated carboxylic acid containing from 3 toabout 5 carbon atoms, preferably acrylic and/or methacrylic acid, saidemulsion polymer having a glass transition temperature of less thanabout -30° C. and a gel content of from about 50 to about 70 percent byweight of the polymer.

Although the emulsion adhesive polymers of the instant invention can beprepared by using only conventional surfactants, it is preferred toadditionally employ a reactive surfactant which polymerizes and becomespart of the emulsion polymer and which has been observed to enhancecohesive strength and aid in copolymerization of the monomers in formingthe emulsion pressure-sensitive adhesive polymers of the instantinvention. If employed, the amount of reactive surfactant employed inthe preparation of the emulsion pressure-sensitive adhesives of thepresent invention is present in an amount up to about 0.4 percent byweight of the total monomers, preferably from about 0.1 to about 0.25percent by weight. The preferred reactive surfactants are anionic vinylfunctional surfactants, such as sodium vinyl sulfonate and sodiumstyrene sulfonate.

The emulsion adhesives of the instant invention may be prepared withexcellent conversions at reaction temperatures ranging from 70° to about85° C. in the presence of from about 0.5 to about 1 percent by weight,based on the weight of the monomers, of a persulfate or equivalentcatalyst, with the monomer mix being fed over a period of about 4 toabout 5 hours. Reaction pH is from about 2.5 to about 4.0. Conversion ishigh, approaching 100 percent at the reaction conditions set forthabove.

The polymers may be modified using cross-linking reactions induced bymetal salts, organometallic complexes, electron beam radiation, actinicradiation, or heat. A significant improvement in cohesive strength canbe achieved without much loss of peel and tack by incorporating smallamounts of chelating monomers having at least one active methylene groupin the pendent chain, with or without a chain transfer agent, andadditionally by crosslinking the polymers using metal salts such asaluminum acetate.

DETAILED DESCRIPTION

The present invention relates to pressure-sensitive adhesivecompositions based on emulsion polymers which provide high adhesion andhigh tack to polar, nonpolar and difficult-to-bond substrates withexcellent cohesion. The adhesives have properties sufficient to makethem useful for replacing solvent polymers, tackified styrene-butadieneemulsion pressure-sensitive adhesives, and nontackified and tackifiedacrylic pressure-sensitive adhesives. An improvement exhibited overtackified styrene-butadiene resin adhesives include better aging and noedge ooze or bleed as part of a release liner adhesive face stocklaminate. In addition, being functional as a single polymer, there is aminimal or no need for compounding and tackification. The improvedperformance characteristics of the adhesive of the instant inventionenable them to be used on almost any available face stock. Propertiesinduced in the adhesive by proper selection of monomers and surfactantsgive excellent moisture resistance enabling the adhesive to be used formedical and moisture-resistant pressure-sensitive adhesive applications.

As compared to prior art acrylic pressure-sensitive adhesives which donot give good adhesion to nonpolar surfaces, such as polyolefins andcertain other surfaces such as recycled corrugated board, withouttackification with attendant sacrifice in cohesive strength andlow-temperature performance, the adhesives prepared in accordance withthe instant invention have excellent adhesion to both polyolefins andcorrugated board and good low-temperature performance. The adhesives aremore universal in their use, a result not heretofore achieved in singlepolymer adhesives.

The emulsion based pressure-sensitive adhesives of the instant inventioncontain, on a percent by weight basis from about 35 to about 60 percentby weight total, one or more alkyl acrylates containing about 4 to about8 carbon atoms in the alkyl groups, and preferably total alkyl acrylateconcentration, including mixtures of alkyl acrylates, preferably presentin a total amount of from about 40 to about 50 percent by weight of themonomers. Useful alkyl acrylates include n-butyl acrylate, 2-ethyl hexylacrylate, isooctyl acrylate, and the like, with 2-ethyl hexyl acrylatebeing preferred.

The second monomeric component is one or more vinyl esters present in atotal amount of from about 15 to about 35 percent by weight, preferablyfrom about 20 to about 25 percent by weight based on total weight of themonomers, said vinyl esters containing from 2 to about 16 carbon atomsin the alkyl group of the acid. Representative of the vinyl estersinclude vinyl acetate, vinyl butyrate, vinyl propionate, vinylisobutyrate, vinyl valerate, vinyl versitate, and the like. Vinylacetate is preferred.

The third component of the emulsion polymers of the instant inventionare one or more diesters of a dicarboxylic acid and mixtures thereof,present in a total amount of from about 20 to about 35 percent by weightbased on the total weight of the monomers. Each ester group of thediester of the dicarboxylic acid independently contains from about 8 toabout 16, preferably from about 8 to about 12, carbon atoms. Thepreferred diesters are di-2-ethyl hexyl maleate, di-2-ethyl hexylfumarate and mixtures thereof.

A fourth component of the instant invention is at least one unsaturatedcarboxylic acid containing from about 3 to about 5 carbon atoms andpresent in a total amount of up to about 5 percent by weight of thepolymer, preferably from 1 to about 3 percent by weight. The unsaturatedcarboxylic acid includes, among others, acrylic acid, methacrylic acid,itaconic acid, and the like. Acrylic acid, methacrylic acid, and morepreferably mixtures thereof, are presently preferred.

The emulsion adhesives of the instant invention are preferably preparedin the presence of a reactive surfactant which polymerizes duringformation of the polymer and becomes an integral part of the polymer.Preferred reactive surfactants include anionic vinyl functional monomerssuch as sodium vinyl sulfonate and sodium styrene sulfonate and thelike. The reactive surfactant is present as part of the total surfactantsystem and in an amount up to about 0.4 percent by weight of the totalmonomers, preferably about 0.1 to about 0.25 percent by weight.

Presently preferred emulsion polymers contain, exclusive of reactivemonomers, about 48 percent by weight 2-ethyl hexyl acrylate, about 21percent by weight vinyl acetate, about 29 percent by weight di-2-ethylhexyl maleate, about 1 percent by weight acrylic acid, and about 1percent by weight methacrylic acid.

The monomer proportions are adjusted in such a way that the adhesive hasa glass transition temperature less than about -30° C., preferably lessthan about -34° C., giving a good balance of adhesion and tack at roomtemperature and low temperatures. The emulsion polymers of thisinvention have a very broad glass transition temperature range of from15° to 30° C., e.g. -22° to -52° C., which is unusual among acrylicpolymers. Conventional acrylic polymers, for instance, have a glasstransition temperature range of only 10° to 15° C. Depending onpolymerization conditions, copolymers showing two distinct glasstransition temperatures, one in the region of -45° to -60° C. and theother in the region of -15° to -30° C., have been observed.

Gel content or percent insolubles are in the range of 50 to 65 percentby weight which provides excellent cohesive strength without the use ofmultifunctional monomers. In this regard, gel represents the amount ofpolymer which is insoluble in tetrahydrofuran expressed in percent byweight and determined by the membrane gel partitioning method. In thismethod, about 600 to 800 milligrams of 100 percent solid polymer isweighed onto a millipore membrane disk of 5 micrometer porosity. Thedisk is heat sealed and transferred to a scintillation vial. About 20milliliters of tetrahydrofuran is added to the vial and the vial isrotated on a shaker for 16 to 24 hours. The sealed disk is then removed,washed with tetrahydrofuran, and dried first by placing it on a WhatmanNo. 1 filter paper, followed by exposure to 100° C. in the oven for afew hours. The dried disk is weighed and the insoluble portion of thepolymer determined by the equation: ##EQU1## wherein a=total weight of100 percent solids polymer

b=the weight of the polymer plus membrane before tetrahydrofurantreatment

c=polymer plus membrane remaining after tetrahydrofuran treatment.

Polymer properties can be further modified to fit end use applicationsby inclusion of multifunctional monomers and the use of other chemicalcross-linking agents. Other aids which may be used to developcrosslinking include thermal cross-linking and cross-linking by actinicand electron beam radiation.

Polymers of the instant invention are prepared by emulsionpolymerization under conditions of agitation in an autogenous atmospherein the presence of suitable polymerization initiators such asperoxydisulfate and peroxides. Depending on desired polymer propertiesincluding gel content, the preferred levels of these initiators are inthe range of from about 0.5 to about 1.0 percent by weight based on thetotal weight of the monomers. The presently preferred initiators arepotassium persulfate, t-butyl hydrogen peroxide, and the like. Level ofagitation will vary depending on the system and will influenceconversion. Typically, about 30 to 50 percent of the total initiator isadded along with an initial monomer charge to the reactor, and the restis added along with the balance monomers during polymerization over aperiod of from about 4 to about 5 hours. For the polymer to be free fromcoagulum and to maintain grit levels less than 20 ppm, it is desirableto maintain the pH of the emulsion during polymerization between fromabout 2 to about 4, preferably from about 2.5 to about 4. This can beachieved by the use of buffers such as sodium bicarbonate and sodiumacetate, typically in amounts up to 0.3 percent by weight based on theweight of the monomer.

The stabilizer system used during polymerization contains a combinationof anionic and nonionic surfactants present in an amount up to about 3.5percent by weight based on the weight of the monomers. A suitableanionic surfactant is the sodium salt of an ethoxylated nonylphenolsulfate, and a suitable nonionic surfactant is ethoxylated nonylphenol.The best balance of properties is achieved by maintaining the anionic tononionic surfactant ratio approximately 3 to 1.

Polymers of the instant invention are produced at high solids levelcontent, typically about 50 to about 70 percent by weight. Reaction iscarried out at temperatures from 70° to 85° C. with an initial charge ofup to about 10 percent by weight of the total monomers, with the balanceof the monomers being added to the emulsion reaction system over aperiod of about 4 to about 5 hours, with total monomer conversionapproaching 100 percent.

It is possible to modify the rheology of the polymer for coatingpurposes by use of conventional thickeners such as SCT-270 manufacturedand sold by Union Carbide and present in an amount up to 0.2 percent byweight. Although not required, it is also feasible to modify theproperties by the use of tackifiers and the like. For general purposeapplications, it is desirable to have good adhesion to both hydrophobicsubstrates such as polyethylene and hydrophilic substrates such asstainless steel and difficult-to-bond recycled corrugated board. Priorart acrylic polymers do not have good adhesion to such substrates unlessthey are modified by addition of a tackifier. This is not required usingthe pressure-sensitive adhesive polymer of this invention althoughtackifiers can be effectively used to tailor adhesion to a substrate.

It is also feasible, in accordance with the present invention, toprovide emulsion pressure-sensitive adhesive polymers with high shearperformance even at elevated temperature with little or no detriment inpeel and tack adhesion which can be used to replace high cost solventacrylic polymers used in film applications. Such polymers can betackified to enhance adhesion to low energy substrates such aspolyolefins even at low temperatures. The dioctyl maleate in thecomposition provides tackiness to the adhesive and allows the use ofvinyl acetate which provides good peel adhesion without the use of highlevels of acid monomer which tends to reduce tack.

Improved shear may be achieved by copolymerization with small amounts,i.e., about 0.1 to about 1 percent by weight of the monomers, offunctional chelating monomers such as acetoacetoxy ethyl methacrylate(AAEMA), which have a pendent chain with at least one active methylenegroup, and which enable complex formation with metal salts. As usedherein, the term "active methylene group" refers to a functional groupcapable of exhibiting ketoenol tautomerism and anion formation.

The presence of the chelating monomer in the copolymer itself providessome improvement in shear performance which may not be sufficient enoughfor high performance film applications. Further cross-linking by complexformation with metal salts dramatically enhances the shear in excess of10,000 min. Elevated temperature (70° C.) shear is also greatlyimproved, equalling the performance of the emulsion polymers to solventacrylic systems. Adding chelating monomers to the copolymer may alterthe gel content and/or glass transition temperature of the adhesive.Accordingly, when chelating monomers are used, they are added in anamount that yields an adhesive that is pressure-sensitive, i.e., theglass transition temperature is sufficiently low as to enable use as aPSA. The mechanism of using chelating monomers is described, forinstance, in U.S. Pat. No. 4,354,008 to Skoltchi, and U.S. Pat. No.4,908,403 to Spada, et al., each of which is incorporated herein byreference.

To maintain good balance of peel and tack with high shear it isdesirable to lower the molecular weight of the polymer by the use ofchain transfer agents such as n-dodecyl mercaptan(n-DDM) and to usehigher levels of initiator than what is commonly used. An optimum levelof n-DDM in this invention is up to about 0.025 percent by weight of themonomers, preferably about 0.005 to about 0.01 percent by weight, and toemploy a level of initiator of about 0.6 to about 0.75 percent by weightbased on the weight of the monomer composition. Amounts exceeding theselevels can result in cohesive failure in peel adhesion on certainsubstrates which will be detrimental to the product application.Polymers with low levels of initiator provide high shear with AAEMA andaluminum acetate (AA) cross-linking, but only with loss of peel and tackadhesion. Aluminum acetate, if present, is normally present in an amountup to about 0.25 percent by weight of the monomers. However, the polymercontaining the chain transfer agent and a higher level of initiatorgives high shear with little or no loss in adhesion properties.

The high shear associated with the modified polymer makes it suitablefor tackification with conventional tackifiers such as Snowtack lineresins sold by Albright Wilson Co. and Aquatac resins sold by ArizonaChemicals expressly to enhance adhesion to low energy substrates such aspolyolefins. For such applications, it is generally necessary to providea glass transition temperature, e.g., below -30° C. The Tg of thepolymer can be as low as -40° C. to promote good adhesion to low energysubstrates at low temperatures as low as -5° C. without sacrificing theroom-temperature properties. This is done by increasing the ratio of thesoft monomer to vinyl acetate in the total composition.

EXAMPLES 1 TO 4 AND CONTROLS 1 AND 2

To a one liter reactor equipped with a reflux condenser, a thermocouple,a pitched turbine agitator and a nitrogen inlet tube, there was chargeda solution containing 75 g of deionized water, 4 g of sodium vinylsulfonate (25 percent w/w solution in water) and 0.36 g of an anionicsurfactant (Alipal CO-433, a sodium salt of ethoxylated nonylphenolsulfate manufactured and sold by GAF Chemical Co.). A monomer mixconsisting of 215 g of 2-ethyl hexyl acrylate, 150 g of di-2-ethyl hexylmaleate, 125 g of vinyl acetate, 5 g of acrylic acid and 5 g ofmethacrylic acid was added to 115 g of water containing 44.63 g ofAlipal CO-433 and 6.44 g of Igepal CO-887, an ethoxylated nonylphenolnonionic surfactant manufactured and sold by GAF Chemical Co., andagitated to make a pre-emulsion. The reactor charge was heated undernitrogen to 70° C., to which was added 24.25 g of a potassium persulfatesolution (3.78 percent w/w solution in deionized water). Sixty-six gramsof the preemulsified monomer and 12.125 g of potassium persulfatesolution were added to the reactor over 20 to 30 minutes. After thetemperature reached a steady state, the remaining monomer pre-emulsionand a 1.8 percent aqueous solution of potassium persulfate buffered withsodium bicarbonate were introduced into the reactor at respective ratesof 2.5 and 0.32 g/min. for a period of about 240 minutes. The reactortemperature was maintained between 79° to 82° C. After the end of feed,the reactor temperature was raised to 83° to 85° C. and maintained for90 minutes. Once the polymerization was complete, the contents werecooled to ambient temperature and discharged. The polymer had 59.27percent solids, 0.01 percent coagulum, Brookfield viscosity (spindlenumber 3 at 12 rpm) of 2500 cps, a final pH of 3.8, and a Tg of -28.5°C.

Using the above procedure the polymers listed in Table 1 were alsoprepared.

                  TABLE 1                                                         ______________________________________                                                                         Percent                                           Monomer Composition                                                                           Weight Ratio                                                                              Solids pH                                    ______________________________________                                        EX.  2-EHA.sup.1 /DOM.sup.2 /                                                                      43/30/25/2  59.7   4.35                                  2    VAc.sup.3 /AA.sup.4                                                      Ex. 3                                                                              2-EHA/DOM/VAc/  48/29/21/1/1                                                                              59.6   3.80                                       AA/MAA.sup.5                                                             Ex. 4                                                                              2-EHA/DOM/VAc/  50/27/21/1/1                                                                              59.3   3.54                                       AA/MAA                                                                   ______________________________________                                         .sup.1 2ethyl hexyl acrylate                                                  .sup.2 dioctyl maleate                                                        .sup.3 vinyl acetate                                                          .sup.4 acrylic acid                                                           .sup.5 methacrylic acid                                                  

Similar polymers were made using the same procedure, but replacingAlipal CO-433 with Polystep B-27, manufactured and sold by StepanChemicals, and replacing Igepal CO-887 with Polystep F-9, alsomanufactured and sold by Stepan Chemicals.

Table 2 compares the adhesive performance of the adhesive of Example 3to commercial pressure-sensitive adhesive (Controls 1 and 2) at roomtemperature. Better overall properties and superior room-temperatureshear were achieved.

                  TABLE 2                                                         ______________________________________                                                Ct.                                                                           Wt.                                                                           g/     90° peel N/m.sup.8                                                                  Loop Tack N/m.sup.9                                                                      RTS.sup.10                                     sqm    SS.sup.11                                                                            PE.sup.12                                                                          CB.sup.13                                                                          SS   PE   CB   Min.                           ______________________________________                                        Ex. 3   22-25  480    330  280  740  520  360  140 c                          Control 1.sup.6                                                                       23-26  310    350  150  650  525  300   30 c                          Control 2.sup.7                                                                       23-26  710 c  230  315 c                                                                              720  460  400   5 c                           ______________________________________                                         .sup.6 Tackified SBR adhesive                                                 .sup.7 Cargill 6441, a commercial polymer sold by Cargill Inc. and            containing the same monomers used in this invention                           c = cohesive failure                                                          .sup.8 PSTC No. 2, 5th Ed.                                                    .sup.9 PSTC No. 5, 6th Ed.                                                    .sup.10 Roomtemperature shear = PSTC No. 7, 6th Ed. (500 g load)              .sup.11 Stainless steel                                                       .sup.12 Treated high density Polyethylene                                     .sup.13 Recycled corrugated board                                        

Tackified SBR (Control 1) shows a lower adhesion to corrugated board andhas a low cohesive strength. Besides this, Control 1 had a tendency toexhibit bleed and edge ooze on aging, which make the adhesive nonusefulwith certain face stocks. The Cargill 6441 (Control 2) lacked cohesivestrength and displayed a cohesive mode of failure for peel adhesion,which make it of questionable acceptability for label applications.

EXAMPLES 5 AND 6

Experiments were performed with and without a reactive surfactant, i.e.sodium vinyl sulfonate (SVS), to establish its effects on shearperformance.

As can be seen from Table 3, Example 5 gives room-temperature shearvalues comparable to Example 3, whereas Example 6, containing no SVS,gives lower shear values.

                  TABLE 3                                                         ______________________________________                                                           90° Peel                                                                        L. Tack                                                   Percent    N/m      N/m      RTS.sup.a Min.                           Sample  Solids     SS       SS       SS                                       ______________________________________                                        Ex. 5   59.8       370      670      130 c                                    Ex. 6   59.8       355      700       58 c                                    ______________________________________                                         .sup.a 500 g load                                                        

There is an apparent improvement in shear for the polymer with the useof SVS. SVS has been observed to also control the viscosity of theemulsion polymer to accommodate high solids and for good conversion.With no SVS, the viscosity is less consistent and is higher than thepolymer prepared using SVS (5000 to 15000 cps vs. 3000 to 5000 cps).

Control 3

Further experiments with the composition of the present invention havebeen carried out to obtain gel content similar to that of the Cargill6441 polymer for a better comparison.

To obtain Control 3, therefore, Example 3 was repeated with thedifference that the preemulsion contained 0.125 g of n-dodecylmercaptan, a chain transfer agent, the total initiator was 5 g and thetotal feed time was 270 minutes. Table 4 provides the comparativeproperties of Examples 1 and 3 to each other and to the Cargill 6441polymer and the polymer of Control 3. Examples 1 and 3 give very goodroom-temperature performance with good shear. The polymer of Control 3,with a lower gel content similar to Cargill 6441, had a very low shearperformance combined with cohesive mode of failure for peel adhesion.

Table 4 shows the low-temperature performance obtained for thepressure-sensitive adhesives of the present invention compared toExample 1 and Control 3 and the Cargill 6441. Example 3 shows goodadhesion and good fiber pick on recycled corrugated board, and goodadhesion and face stock tear on treated high density polyethylene.Example 1, with higher Tg, had better room-temperature performance thanlow-temperature performance. Control 3 and Cargill 6441, with lowcohesion, showed only fair adhesion.

                  TABLE 4                                                         ______________________________________                                        Low-Temperature Performance Properties                                                                     25° Peel                                         Ct. Wt.               Adhesion.sup.13                                  Period   g/sqm   % Gel   Tg, °C.                                                                      RCB    TRHDPE                                  ______________________________________                                        Ex. 1    26      53      -28   F-P/CL F/CL                                    Ex. 3    23      58      -34   G/GFP  G/SlFT                                  Control 3                                                                              23      25      -36   F/SlFT F/CL                                    Cargill 6441                                                                           23      20      -27   F/VslFP                                                                              F/CL                                    ______________________________________                                         .sup.13 practical peel adhesion determined by subjective hand testing         RCB = recycled corrugated board                                               TRHDPE = treated high density polyethylene                                    G = good adhesion                                                             F = fair adhesion                                                             P = poor adhesion                                                             CL = removes clean                                                            GFP = good fiber pick                                                         VslFP = very slight fiber pick                                                SlFT = slight face tear                                                  

EXAMPLES 7 TO 11 AND CONTROLS (CTL) 4 TO 6

To a one liter reactor equipped with a reflux condenser, a thermocouple,a pitched turbine agitator and a nitrogen inlet tube, there was chargeda solution containing 75 g of deionized water, 2 g of sodium vinylsulfonate (25 percent solution in water) and 0.36 g of Polystep F-27emulsifier. A monomer mix consisting of 237.5 g of 2-ethyl hexylacrylate (2-EHA), 145 g of dioctyl maleate (DOM), 105 g of vinyl acetate(VA), 7.5 g of acrylic acid (AA), 2.5 g of methacrylic acid (MAA), 2.5 gof acetoacetoxy ethyl methacrylate (AAEMA), and 0.25 g of n-dodecylmercaptan (n-DDM) was added to 115 g of water containing 44.63 g ofPolystep B-27 and 6.44 g of Polystep F-9 and agitated to make apre-emulsion. The reactor charge was heated to 72° to 75° C. and therewas added 24.25 g of 4.37 percent w/w of potassium persulfate solution.Sixty-six grams of the pre-emulsified monomer and 12.13 g of thepotassium persulfate solution were added over 20 to 30 minutes. Afterthe temperature reached a steady state, the remaining monomerpre-emulsion and a 2.21 percent aqueous solution of potassium persulfatebuffered with sodium bicarbonate were introduced into the reactor atrespective rates of 2.22 and 0.29 g/min. over a period of 270 minutes.The reaction temperature was maintained between 79° to 82° C. After theend of feed, process temperature was raised to 83° to 85° C. Thirtyminutes after the feed, 5.25 g of a 4.8 percent solution of potassiumpersulfate was added and the reactants maintained for 90 minutes. Oncethe polymerization was complete, the contents were cooled to ambienttemperature, neutralized with ammonia to pH 5.5 to 6.5, and discharged.The polymer content was 60 percent solids, with less than 0.02 percentcoagulum. Polymer Tg was -35° C.

Using the same procedure, the polymers listed in Table 5 were prepared,except for examples 10 and 11 where the initial charge had 3.85 percentsolution of initiator and the main feed had 1.9 percent solution ofinitiator.

                  TABLE 5                                                         ______________________________________                                                                            Percent                                   Example                                                                              Monomer Composition                                                                           Wt. Ratio    Solids                                    ______________________________________                                        Ex. 8  2-EHA/DOM/Vac/  48/29/21/0.005/                                                                            59.0                                             n-DDm/AA/MAA    1.5/0.5                                                Ex. 9  2-EHA/COM/VAc/  47.5/25/17/0.5/                                                                            59.5                                             AAEMA/AA/MAA/   1.5/0.5/0.005/8                                               n-DDm/BA                                                               Ex. 10 2-EHA/DOM/VAc/  48/24/16/10/1.5/                                                                           58.9                                             BA/AA/MAA/n-DDM 0.5/0.005                                              Ex. 11 2-EHA/DOM/VAc/  48/29/20.5/1/                                                                              59.3                                             AA/MAA/AAEMA    1/0.5                                                  ______________________________________                                    

Table 6 shows the adhesive properties of Examples 7 to 11 against, forpurposes of these Examples, controls which are polymers made by theprocedure of Example 11 with similar composition but not containingn-DDM or AAEMA. As in all other examples, shear was measured using asample measuring 1/2"×1/2". Shear values (both RTS and 70° C. shear)were determined using a 500 g load, and measured in minutes. Thepolymers of Examples 7 to 11 show better overall properties. Example 2and 10, with no AAEMA and with only ionic cross-linking with aluminumacetate (AA), gave shear values much less than Example 7. Example 11,containing no n-DDm and polymerized with a lesser level of initiator,had a good shear value, although the adhesion properties were lower. Inthe Controls, the improvement of shear is not dramatic, and justcross-linking with aluminum acetate reduces adhesion values. Acombination of chain transfer agent (n-DDM) and the higher level ofinitiator give low molecular weight polymers which provide good adhesionvalues and high shear.

                  TABLE 6                                                         ______________________________________                                                  AA              90°                                                                         Loop        70° C.                      Ct. Wt.   Wt.    AAEMA    Peel Tack RTS,   Shear                              g/sqm     %      Wt. %    N/m  N/m  Min.   Min.                               ______________________________________                                        Ex. 7 23      0.06   0.5    295  510   10000+                                                                              4000+                            Ex. 8 23      0.15   --     295  480  1600   --                               Ex. 9 23      0.15   0.5    245  310   10000+                                                                              --                               Ex. 10                                                                              23      0.15   --     250  353  2300   --                               Ex. 11                                                                              28      0.06   0.5    240  400   7000+ --                               Ctl. 4                                                                              21      0.06   --     250  320   600   --                               Ctl. 5                                                                              23      0.20   --     250  340  2200   --                               Ctl. 6                                                                              27      --     --     330  540   210   --                               ______________________________________                                         Release liner = 1100                                                          Face stock = 2 mil Mylar                                                 

EXAMPLES 12 TO 19

Using the general procedure described above, additional emulsionpolymers were made by varying the amount of monomers and acetoacetoxyethyl methacrylate, as shown in weight percent in Table 7. The initiatorlevel was maintained between 0.6 and 0.7% based on the total monomerweight, and the n-dodecyl mercaptan level was maintained below 0.015% byweight.

                  TABLE 7                                                         ______________________________________                                                                                    Other                                  2-                                     Mono-                             Ex.  EHA    VAC     DOM   AA   MAA   AAEMA  mers                              ______________________________________                                        12   47.75  21      29    1.5  0.5   0.25   --                                13   47.9   21      29    1.5  0.5   0.1    --                                14   48     21      28    2    1     0.15   --                                15   43     25      29    2    1     0.15   --                                16   48     20      28    1.5  1.5   0.1    1.0 t-                                                                        octyl                                                                         acryl-                                                                        amide                             17   43     25      27.55 1.5  0.5   0.15   0.3                                                                           amide,                                                                        2 HPA                             18   43     25      29.65 1.5  0.5   0.15   0.2                                                                           MAEEU                             19   52.4   20      24.85 2    0.75  0.15   --                                ______________________________________                                         Amide = Acrylamide                                                            HPA = Hydroxypropyl acrylate                                                  MAEEU = Methacrylamide of aminoethyl ethylene urea                       

The emulsion polymers were coated and dried, and tested using polyesterfacestock in pressure-sensitive adhesive construction. Table 8 gives theinitial and one week at 70° C. aged performance data on a stainlesssteel test panel. All of the polymers show good adhesion with highcohesion. Even the polymers containing low levels of acetoacetoxy ethylmethacrylate (e.g., example 13) show shear values in excess of 6000minutes. The plus sign after the shear values indicate that the sampleswere removed after that time and that the test was discontinued.

                  TABLE 8                                                         ______________________________________                                                180° Peel N/m                                                                     Loop Tack N/m                                                                              RTS                                                 Ct. Wt.         1/Wk,        1/Wk,  500 g                               Ex.   g/sqm   20 Min  @70° C.                                                                       Init. @70° C.                                                                       Min.                                ______________________________________                                        12    25      450     350    485   330    10000+                              13    27      570     --     485   --      6000+                              14    30      450     350    600   360     6000+                              15    25      470     380    600   370     5000+                              16    25      520     445    500   500    2000-                                                                         4000                                17    25      420     355    390   235    --                                  18    24      455     480    480   440    10000+                              19    24      350     --     425   --     10000+                              ______________________________________                                    

What is claimed is:
 1. The process for the production of inherentlytacky, pressure-sensitive adhesive emulsion polymers whichcomprises:subjecting an emulsified mix of monomers comprising:(i) atleast one alkyl acrylate containing from about 4 to about 8 carbon atomsin the alkyl group and present in a total amount of 35 to about 65percent by weight of the monomers; (ii) at least one vinyl estercontaining from about 2 to about 16 carbon atoms in the alkyl chain ofthe acid, the total of the vinyl esters being present in an amount of 15to about 35 percent by weight of the monomers; (iii) at least onediester of a dicarboxylic acid present in a total amount of from about20 to about 35 percent by weight of the monomers, each diesterindependently containing from 8 to about 16 carbon atoms in the alkylgroup of the ester; and (iv) up to about 5 percent by weight of themonomers of at least one unsaturated carboxylic acid; (v) a positiveamount of a celating monomer having a pendant chain which contains atleast one active methylene group to emulsion polymerization byinitiating polymerization of an initial monomer mix consisting of aportion of the total monomers in the presence of an initiator, andadding the balance of the monomers and the initiator therefore to theemulsion reaction system over a period sufficient to achieve a totalsolids content up to about 60 percent by weight of the emulsion, saidemulsion reaction being maintained under autogenous conditions at atemperature of about 70° to about 85° C. and a pH of from about 2 toabout 4 in the presence of a stabilizer system comprising a reactivesurfactant which is a vinyl function monomer and an anionic and anonionic surfactant system, in which the anionic-to-nonionic surfactantratio is approximately about 3 to about 1, said polymer having a glasstransition temperature of less than about -30° C. and having a gelcontent of about 50 to about 70 percent by weight.
 2. A process asclaimed in claim 1 in which the emulsion is carried out at a pH of fromabout 2.5 to about
 4. 3. A process as claimed in claim 1 in which thereactive surfactant is selected from the group consisting of sodiumvinyl sulfonate and sodium styrene sulfonate.
 4. A process as claimed inclaim 1 in which the chelating monomer is present in an amount of fromabout 0.1 to about 1 percent by weight of the monomers and in which achain transfer agent is present in an amount of from about 0 to about0.25 percent by weight of the monomers.
 5. A process as claimed in claim4 in which the chelating monomers is acetoacetoxy ethyl methacrylate. 6.A process as claimed in claim 4 in which the chain transfer agent isdodecyl mercaptan.
 7. A process as claimed in claim 6 in which thechelating monomer is acetoacetoxy ethyl methacrylate.